Slack, minus the ability to gracefully run in any sort of long term roaming context without babysitting. There's no decent reconnecting logic on either client or server, you just have to notice it's gone quiet, and then figure out the undocumented way to restore it without nuking your settings. Only figured out by accident "/connect" without parameters is valid.
I tried to fix it, but the client is a pile of terrible code and needs a complete rewrite.
Well, if you get disconnected and don't get reconnected, the server tab mentions to try `/connect`.
Additionally, if you want to rewrite the client, you're welcome to start and open a PR - it's a community-run project. I've had Lounge running for months now without issue. Contributions are always welcome.
(Warning: bias/plug; I contribute to Lounge)
In terms of OSS alternative to slack, I recommend you check out Lounge, a self-hosted JavaScript web client (think: selfhosted IRC version of Slack, minus/plus a few features)
https://github.com/thelounge/lounge